Close out state CTC federal-conformance audit (NC/CO/UT)

[DTrim99]

What

Closes out the state CTC ↔ federal-conformance audit (#6374) by resolving its three remaining watch-items. Two are verification/regression-only (no behavior change); one adds a missing reference.

NC — regression test (no code change)

The audit's open question was whether NC's "allowed the federal §24 credit" language re-imports the OBBBA (July 2025) filer-SSN gate. It does not: NC is a static IRC-conformity state, conformed to the Code as of Jan 1, 2023 (N.C. Gen. Stat. §105-228.90(b)(1b)), and NCDOR's TY2025 guidance confirms NC does not conform to OBBBA for 2025. §24's child-SSN rule (TCJA, 2018) is in the Jan-1-2023 Code and is correctly imported via ctc_qualifying_children; the filer-SSN rule (2025) is not. This PR adds a regression test: a 2025 ITIN filer whose qualifying child has a valid SSN still receives the NC child deduction. It fails if a filer-SSN gate is ever propagated into the NC count.

CO — boundary test (no code change)

co_ctc selects between rate × co_federal_ctc (2022–2023) and a flat per-child amount (2024+) via ctc_matched_federal_credit (true → false at 2024-01-01). The existing unit tests jumped from 2022 to 2024, leaving the flip date unguarded. Verified the 2022 tests correctly exercise the rate branch (no mismatch — the watch-item was a false alarm), and added a 2023 case pinning the rate branch is still active in 2023.

UT — missing reference

ut_ctc_potential.py had no reference. Added Utah Code §59-10-1047 (the statute the surrounding parameters already cite).

Testing

• NC nc_child_deduction.yaml: 7 passed
• CO co_ctc.yaml: 28 passed
• UT ut_ctc.yaml: 14 passed

Fixes #6374.

The two remaining audit notes are indefinite watch-items with no action needed today: GA/ID cite §24(c) (the definition subsection, silent on SSNs) with no evidence either state intends ITIN eligibility — revisit only if a DOR confirms coverage.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (db70094) to head (a221a32).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #9154    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           17         1    -16     
  Lines          279        24   -255     
==========================================
- Hits           279        24   -255     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.